Lithium niobate (LN) and lithium tantalate (LT) are widely used in a variety of electronic applications including surface acoustic wave (SAW) signal processing, guided-wave optic modulation and switching, and electro-optic laser Q-switching, and modulation. The physical basis for the suitability of LN and LT crystals for these types of applications is their atomic-scale crystal structure, which results in the crystals' natural piezoelectric response useful in SAW-based devices, electro-optic response useful in integrated optic devices and pyroelectric response useful in pyroelectric detectors. Another characteristic of LN and LT that may be important in some applications is the optical absorption of the crystal. For example, integrated optic devices require a relatively small optical absorption, while other devices; such as SAW filters do not require a low optical absorption. In some instances, this natural physical response of the crystals can complicate crystal processing and adversely effect performance of devices in which the crystals are incorporated.
A crystal's pyroelectric or piezoelectric response causes the external surfaces of a fabricated crystal to become electrically charged in response to a change in temperature of the crystal or in response to a mechanical stress applied to the crystal. These electrical surface charges can spontaneously short, with associated sparking causing dramatic processing or performance failure, or even crystal fracture. One common example of such performance failure is an unacceptably high bit-error-rate of LN-based SAW filters used in digital radio applications. In order to avoid such types of failures, current production protocols for these types of filters include extensive and costly device testing designed to eliminate those filters prone to such spurious pyroelectric or piezoelectric induced failures.
The process of incorporating LN or LT crystals into electronic devices often includes steps that result in exposing the crystals to conditions that invoke an untimely and unwanted pyroelectric or piezoelectric response. In an effort to reduce the risk of problems associated with the unintended build up of surface charges, for example catastrophic discharge of these charges during manufacture, device manufacturers have had to take steps that add significantly to the cost, time, and complexity of incorporating the crystals into devices.
For LN and LT crystals manufactured by conventional methods, surface charges can eventually decay with time as they are neutralized by the movement of free charge from within the crystal itself or from the surrounding environment. This natural decay occurs after the surface charge develops and does nothing to mitigate or minimize the degree to which the surface of the crystal becomes charged as a result of the crystal's natural piezoelectric or pyroelectric response.
In view of the increasing demand for reliable LN and LT crystals for applications such as surface acoustic wave filter devices, guided wave optic modulation and switching, and electro-optic Q-switching and modulation, the need exists for LN and LT crystals that continue to exhibit properties that make them desirable for such applications and that do not suffer from the drawbacks associated with the buildup of excessive spurious pyroelectric or piezoelectric surface charges.